Nonlocal wind‐driven fjord–coast advection and its potential effect on plankton and fish recruitment

Abstract

The Bergen Ocean Model (BOM), a three‐dimensional physical coastal ocean model, was used for a numerical simulation experiment to investigate short‐term effects of wind‐generated coastal upwelling and downwelling on the dynamics of adjacent large outer and smaller inner fjords. The effect of the real alongshore wind regime on advection for an idealized fjord topography, resembling Masfjorden, western Norway, is used as an example. This modelling exercise is a supplement to, and its predictions support, the various hypotheses investigated in ecosystem simulation studies of the Masfjorden. The model predicts that coastal winds from the north cause upwelling and transport the upper water layer out from the fjords. Winds from the south cause downwelling and transport the upper water layer into the fjords. The transport is rapid and ≈50% of the upper water layer may be replaced within 1–2 days. Implications of these physical processes for the dispersal and retention of planktonic organisms and the early life stages of fish are discussed. If strong southerly winds occur frequently, this will transport planktonic organisms into the fjord and may increase the carrying capacity for planktivorous fish. In contrast, frequent strong northerly winds may reduce the abundance of planktonic organisms, including the early life stages of marine fish, and thus possibly reduce recruitment to fjord fish populations. Frequent shifts between southerly and northerly winds would cause an exchange of early life stages between neighbouring fjords and thus enhance genetic exchange.